DESCRIPTION (Applicant's Description): The long-term objective of this proposal is to understand how transcriptional programs underlying organ-specific patterns of development are initiated. The experimental system is floral patterning in the model plant Arabidopsis, which is particularly suited for genetic and molecular studies because of its short generation time, small genome, and facile DNA-mediated transformation. The specific problem to be investigated is how the LEAFY transcription factor interacts with other factors to regulate flower-specific gene expression. By comparing the processes of pattern formation in animals and plants -- which evolved multicellularity independently -- universal principles underlying these processes can be uncovered. The health relevance of such knowledge relates to birth defects resulting from aberrant pattern formation caused by environmental effects, and to genetic disorders disrupting genes encoding developmental regulators. LEAFY, which is the earliest known transcription factor to be specifically activated in floral primordia, is both necessary and sufficient for converting vegetative shoots into flowers. A unique feature of LEAFY compared to animal transcription factors is that it can be exported to adjacent cells, where it can activate direct target genes. One class of direct target s, homeotic genes that control floral organ fate, has been previously identified, and includes the AGAMOUS gene. However, other targets besides homeotic genes must be required for flower formation, since flowers are distinguished from shoots not only by differences in organ identity but also in organ arrangement. Experiments are proposed (1) to study AGAMOUS regulation as a proxy for transcriptional control during plant development; (2) to reveal the global expression program induced by LEAFY with the goal of uncovering new classes of target genes; and (3) to identify the cis- and trans-requirements for LEAFY movement with the goal of elucidating this unique mode of cell-cell communication. The proposed experiments will advance our understanding of how widely expressed, early acting genes participate in the regulation of later-acting genes that are expressed only in restricted regions of an emerging organ primordium. The experiments will also reveal how different stem cells communicate to effect coordinated development of the flower.